1. Field of the Invention
The present invention relates to a phase-shift mask and a method of using the phase-shift mask to form a pattern. More particularly, the present invention relates to a method for forming a pattern that is capable of solving phase conflict.
2. Description of the Prior Art
Lithography processing remains an essential step in semiconductor manufacturing today. In the integrated circuit making processes, lithography plays an important role in limiting feature size. By using lithography, a circuit pattern can be precisely transferred onto a die or wafer. Typically, to implement the lithography, a designed pattern such as a circuit layout pattern or a contact layout pattern in accordance with a predetermined design rule is created on one or several mask in advance. The pattern on the mask is then transferred onto the wafer with a stepper and scanner. In general, a photosensitive material, such as photoresist, is coated over a top surface of a die or wafer to selectively allow for the formation of the desired geometries, features, lines, or shapes.
As the feature size gets smaller, it becomes critical to improve the resolution of the photolithographic process in semiconductor manufacturing. One approach to improving the resolution is using short-wavelength light, as, theoretically, the shorter the wavelength the higher the possible resolution of the pattern. This method, though simple, is not feasible. Light sources for providing light of short wavelengths are not accessible. Besides, damage to equipment is considerable when short-wavelength light is used in an exposure process, thereby leading to a shortened equipment lifetime. The cost is thus raised, which makes products not competitive. Due to these conflicts between theory and practice, manufacturers are researching ways of overcoming these problems.
In current Resolution Enhancement Technology (RET), phase shift masks are one of the most critical tools for enhancing resolution. Generally speaking, because the phase of light is not shifted when light passes through traditional masks, some light will form constructive interference on the surface of the wafer, resulting in exposed silent patterns that are not supposed to be formed on the wafer under exposure, thereby leading to a worse resolution of pattern.
As known in the art, phase shift masks are masks with additional phase shifters selectively positioned between the metal Cr line patterns. When the light passes through the phase shifter of the phase shift masks, the phase of the electric field of the light is shifted exactly 180°, so the phase difference between the incident light and the shifted light is exactly half a wavelength and destructive interference therefore occurs on the wafer. The interference effect of the diffraction is resolved by the destructive interference and the resolution of the border of the metal lines is thereby enhanced.
The RET of the phase shift masks still has some disadvantages, however. For example, the distance between the line patterns on the phase shift masks is shortened due to reduced feature sizes. This occasionally leads to phase conflict problems. FIG. 1 illustrates an exemplary layout of conventional line patterns on a Cr-less phase shift mask. The phase shift mask 10 includes a glass substrate 11 and patterns 12. The patterns 12 are formed by the alternate arrangement of the first phase-shift regions 13 and the second phase-shift regions 14. When a pitch between the patterns 12 on the phase shift mask 10 is too short due to critical dimension (CD) shrinkage (35 nm for example), a disadvantageous interconnect 16 occurs on the terminal section of the first phase-shift regions 13 and the second phase-shift regions 14 at the border 15 near the phase inversion due to the phase conflict, as shown in FIG. 2. The interconnect 16 causes the line patterns, which should be independent of one another, to connect with each other, forming so-called annular defects and therefore compromising the correctness and the independence of the following patterns.
It is therefore necessary to provide a novel technology for phase-shift masks to solve this problem.